The White House was under mounting pressure to explain why it did not heed prior warnings about its troubled online healthcare exchange after the release of an expert report predicting many of the problems that marred the launch.

Management consultants McKinsey warned in March that the marketplaces were vulnerable to failure because of the “limited end-to-end testing prior to launch” and the fragility of a system that required every complex element to work perfectly before customers could enrol for health insurance.

The report, released on Monday night by Republican critics on the House oversight committee, appears to confirm growing evidence that the administration turned a blind eye to potential weaknesses during the fevered political atmosphere that preceded the launch on October 1.

Barack Obama insisted last week that he had no knowledge of the internal warnings about the website's reliability before the launch, and denied any political motive for covering-up failures.

“I don't think I'm stupid enough to go around saying: 'This is going to be like shopping on Amazon or Travelocity,' a week before the website opened if I thought it wasn't going to work,” he said.

"We would not have rolled out something knowing that it wasn't going to work the way it was supposed to – given all the scrutiny we knew would be on the website."

But the McKinsey report follows the release of emails and documents showing that many senior staff were aware of the problems in the days and weeks before October 1.

In another document, healthcare.gov project manager Henry Chao said: “I just need to feel more confident they are not going to crash the plane at takeoff.”

Critics on the House oversight and investigations subcommittee claim the evidence shows a president either dangerously out of touch or so desperate not to heed Republican calls for the launch to be delayed that he was prepared to take risks with the website.

Chairman Tim Murphy said: “The administration was under no obligation to launch the website on October 1, yet did so anyway despite the government’s own programmers warning that the site was full of bugs, security holes, and well behind schedule,”

The continued postmortem over the bungled rollout comes as Obama reaches out to his grassroots supporters to win over a sceptical public about the merits of the wider Affordable Healthcare Act, known as Obamacare.

In a conference call on Monday night organised by his former election campaign group, Organising For Action, the president called on an estimated 200,000 supporters to help encourage uninsured Americans to sign up for the new health policies.

“I’ve got one more campaign in me, and that’s making sure this law works,” he said.

“This law is going to be one that lasts for generations to come,” he said. “And people will see why we fought so hard to do it.”

Figures released last week showed on 26,794 Americans have so far signed up through the federally-organised health exchange, with a further 76,319 enrolling through 14 state-run marketplaces.